The question they asked was: What is it in our neural system that prevents us from knowing when we’ve had enough? To find out, the researchers measured enkephalin levels in rats. Enkephalin is an opium-like chemical naturally occurring in the neostriatum, an area of the brain supposedly related to craving.

In the first step, the rats were offered unlimited amounts of M&Ms, while their enkephalin levels were monitored. When they started to eat, enkephalin levels surged. In the second step, the researchers injected synthetic enkephalin into the neostriatum to determine whether the chemical might actually cause the rats to eat more. The results were astonishing. With the stimulation, the rats ate twice as many candies as they did before.

“They ate the equivalent of a 150-pound human consuming seven pounds of M&Ms,” said DiFeliceantonio, who is a Ph.D. candidate in biopsychology at the University of Michigan.

But there was more to find out.

“We then asked whether the injection was making the rats just want to eat more or actually making the M&Ms taste better.”

Through a test in which lip-licking is used as an indicator, the researchers found that while the rats ate more, they didn’t like the M&Ms any more than before.

“So, enkephalin in this area is a purely motivational signal saying, ‘Eat more now!’ ” DiFeliceantonio explained.

“This means that the brain has more extensive systems to make individuals want to over-consume rewards than previously thought,” she said in the Smithsonian article. “It may be one reason why over-consumption is a problem today.”

Alexandra Gold diFeliceantonio ’08 is a Ph.D. candidate at the University of Michigan.

The study may also explain some of the underlying mental reasons behind other addictions, the magazine notes.

“It seems likely that our enkephalin findings in rats mean that this neurotransmitter may drive some forms of over-consumption and addiction in people,” DiFeliceantonio said.

She began the study two years ago in collaboration with several chemists, as well as pharmacologist Omar Mabrouk and his mentor, Robert Kennedy, who measured the enkephalin levels using mass spectrometry. Kent Berridge, DiFeliceantonio’s current mentor and dissertation advisor, is the final author of the study.

DiFeliceantonio was accepted into the psychology program at the university immediately after graduating from Sweet Briar in 2008 with a double-major in psychology and Spanish. She will complete her Ph.D. next spring and is looking for a post-doctoral research position.